function move_zombies()
	local occupied = {}
	for z in all(zombies) do
		last_x = z.x
		last_y = z.y
		if x > z.x then z.x += z.speed end
		if x < z.x then z.x -= z.speed end
		if collide(z.x,z.y,0) then z.x = last_x end
		if y > z.y then z.y += z.speed end
		if y < z.y then z.y -= z.speed end
		if collide(z.x,z.y,0) then z.y = last_y end
		if hit(z.x,z.y,8,8,x,y,8,8) then
				stop('dead')
		end
		for o in all(occupied) do
			if hit(z.x,z.y,8,8,o.x,o.y,8,8) then
				z.x = last_x
				z.y = last_y
				break
				end
		end
		add (occupied,{x=z.x,y=z.y})
	end
end